If you are having graphical glitches in the way that avatars and such appear chopped up or with holes in them, Paquin suggested an apparently successful remedy:
Go to the Start menu, click Run, type in Regedit, and press enter.
In the folder tree, open up HKEY_LOCAL_MACHINE\SOFTWARE\PlalyonlineUS\SquareEn ix\FinalFantasyXI.
You will want to make a backup, just in case something goes wrong, so right click the FinalFantasyXI folder and click export. Save it as whatever you want, and you will have the backup. To restore it, just open up the file like any other and it will do its work.
Next, double click each of the 0003 entry in the box to the right. Click the radio button under Base that says Hexadecimal. Next just change the value in the Value data box from 400 to 500. Hit ok then do the same thing for the 0004 entry. That should fix the problem and probably make your graphics look better too.